1. Field of the Invention
The present invention relates generally to methods and systems for tracking the location of devices near the surface of the Earth, and more particularly to a method and system for tracking a device near the surface of the Earth using a very fast acquisition portable transponder.
2. Background of the Invention
Various techniques are available to determine the position of a device on the surface of the Earth. U.S. Pat. No. 3,063,048 discloses the use of the Doppler shift in a transmitted signal to determine the position of a device transmitting the signal. This patent teaches measuring when the Doppler shift frequency is changing at a maximum rate to determine the position of the object transmitting the signal undergoing the Doppler frequency shift. In this system, the satellite must continuously receive the signal being transmitted from the object to determine when the frequency is changing at its maximum to locate the object. As a result, the time to compute a position fix is unacceptably long for applications such as locating a satellite telephone.
The Global Positioning System (GPS) also provides geo-location capability. Moreover, it is desirable to know the position of a mobile terminal, such as a cellular telephone, and to have this position information at a central location (e.g., at a Service Operations Center or SOC).
One technique is to place a GPS receiver on the mobile terminal, calculate position in the GPS receiver, and transmit via satellite (or other communications system) the position fix to a central location. This method suffers from the Time-To-First-Fix (TTFF) limit inherent in GPS receivers upon waking up from a cold start. From the cold start state, a GPS receiver must download the GPS satellite almanac, ephemeris, and clock correction information. The TTFF limit effectively eliminates using a GPS receiver in situations where a long TTFF is unacceptable. For example, in wireless or cellular telephone telephony applications, it would not be acceptable to require the user to wait for the mobile terminal (i.e., the wireless or cellular telephone) to download GPS data prior to making a telephone call.
The present invention is therefore directed to the problem of developing a system and method for calculating the position of a mobile terminal, which can be accomplished rapidly using a minimal amount of power and equipment.
The present invention solves this problem by collecting observation data at the mobile terminal, forwarding the observation data to a central location, and performing the position calculations at the central location rather than at the individual mobile terminal. Thus, the mobile terminal of the present invention needs only to gather a few milliseconds of observation data, such as GPS code phase information, and then relays this observation data to a central station, via satellite or other relay means, for subsequent position calculation. In this manner, the present invention requires that the mobile unit be on only for very brief periods of time, so that a very fast (e.g. 1 second including data transmission time) position solution can be obtained. This would make practical, for example, the geolocation of a phone prior to a call being placed, which is particularly useful in satellite based phone services to control access and call routing decisions. The fast solution and minimal battery drain make practical the employment of a geolocating system in situations that may not otherwise economically justify such use.
According to one aspect of the present invention, a system for determining the location of an object located on or near the surface includes a communication satellite, a satellite gateway or earth station, a mobile terminal, and a service operations center. In this system, the communications satellite broadcasts an interrogation signal, to which the mobile terminal responds. The mobile terminal is disposed on the object, and includes a receiver, a transmitter and a processor. The receiver receives the interrogation signal from the communications satellite, and is capable of receiving signals being broadcast from GPS satellites. The processor measures at least one characteristic in each of the signals being broadcast from the GPS satellites upon receipt of the interrogation signal from the communications satellite. The mobile terminal transmits a reply signal at a predetermined time relative to receipt of the interrogation signal to the communications satellite. The reply signal includes the measured characteristic(s) in each of the signals being broadcast from the plurality of GPS satellites. The operations center receives the reply signal from the communications satellite, and calculates a position of the mobile terminal using time of arrival information and the measured characteristics returned by the mobile terminal.
In the above system, the measured characteristic(s) can include, for example, code phase information, carrier phase information, Doppler-shift information, or bit phase information. Code phase measurements alone are insufficient to unambiguously identify the position of the mobile terminal. The service operations center requires additional information to constrain the position solution. One means for constraining the position solution is to determine the range from the terminal to the communications satellite.
For this reason, in the first aspect of the present invention, the service operations center includes a processor that calculates a range between the communications satellite and the mobile terminal. In this calculation, the processor uses a time the reply signal arrived at the satellite gateway and a time difference between the broadcast of the interrogation signal to the mobile terminal and its receipt at the satellite gateway. To do so, the processor accounts for known delays in the signal path between the communications satellite and the mobile terminal and the point at which the time-of-arrival information is measured at the satellite gateway. The processor also calculates an intersection curve between a sphere, whose radius is the range previously determined, and a model of the surface of the Earth. Furthermore, the processor determines several initial points on the intersection curve, one of which points must lie within a known convergence zone around the terminal. Next, the processor calculates candidate position solutions for each initial point, and then screens the candidate solutions using predetermined criteria and discards any candidates not satisfying the screening. Finally, if more than one candidate solution remains, the processor selects the solution representing a best fit of all the observation data.
In the above system, the predetermined criteria may include one or more selected of the following: a solution range to the communications satellite, solution residuals, a solution altitude, a solution clock bias and a solution proximity to a beam boundary.
Not all types of communications systems provide the ability to measure propagation delay from a known site such as the satellite. A second preferred embodiment of the present invention solves the problem existing in the art by sending additional signal-related information as data to the SOC along with the code phase measurements. In the second preferred embodiment of the present invention, the additional signal-related information is a signal characteristic such as observed carrier frequency or observed Doppler shift of the carrier frequency. In addition to the measurement of the signal characteristic, in the preferred embodiment of the present invention, the time that the signal characteristic is measured is sent to the SOC. Alternatively, the time that the signal characteristic is measured is estimated from the time the message containing the observed data is received by the SOC. The code phase measurements are obtained at the GPS receiver, for example, in a cellular telephone. These measurements are sent to the SOC. In addition, the carrier frequency of the GPS signal sent by each of the GPS satellites is measured and sent to the SOC. The set of carrier frequency measurements form a set of observed carrier frequencies.
Using the carrier frequencies and a general region containing the location of the receiver candidate locations are determined. The candidate locations are determined by searching in the general region for points in the general region that are consistent with the observed carrier frequencies. From this set of candidate locations, the position of the mobile terminal is determined using the code phase measurements. The position is that candidate location that minimizes a set of residuals calculated using the code phase measurements. It can be seen that the second preferred embodiment of the present invention does not require a satellite (other than the GPS satellites) for operation.
Thus, one object of the present invention is to determine a location of a mobile terminal using a SOC.
Another object of the present invention is to reduce the power requirements of a mobile terminal in order to determine its position.
Another object of the present invention is to reduce the time required for a mobile terminal to be powered on so that its position can be determined.
Another object of the present invention is to increase the speed of acquisition of information required to determine the position of a mobile terminal.
Another object of the present invention is to perform calculations required to determine the position of a mobile terminal on a device other than the mobile terminal.
Another object of the present invention is to provide a location of a mobile terminal using a SOC without having control of the communication channel over which measurement data is sent from the mobile terminal to the SOC.
Another object of the present invention is to provide a location of a mobile terminal using a SOC with little or no change to the communication channel over which measurement data is sent from the mobile terminal to the SOC.
Another object of the present invention is to provide location of a mobile terminal by performing position calculations in a SOC and not in the mobile terminal.
Another object of the present invention is to provide a ground-based (i.e., terrestrial) technique for determining position of a mobile terminal using GPS satellite measurements.
These and other objects of the present invention are described in greater detail in the detailed description of the invention, the appended drawings and the attached claims.